Copiers, facsimile machines and image scanners convert visual images into an electronic form suitable for printing, storage, transmission, or other computer and electronic uses. A system typically includes a light source, an array of photosensors, and electronics for converting analog photosensor outputs into digital data. Light is reflected off of an opaque image medium or light is transmitted through a transparent image medium and focused onto the photosensors. Color devices may have multiple light sources, each with a different band of wavelengths, or broad spectrum light may be split into multiple bands by a color separator, or through the use of filters.
A photosensor array segment includes a linear array of photodetector elements (referred to herein simply as photodetectors) and the electronics to support the photodetectors. The individual photodetectors are spaced a constant distance from each other in the linear array and are typically mounted to, or are part of a printed circuit board or substrate. Each individual photodetector generates a data signal corresponding to the intensity of light the photodetector receives. The data signals generated by the photodetectors are typically voltages. Each data signal generated by each photodetector represents one bit of the data generated by the multi-segment linear photosensor assembly, which, in turn, is one bit of the data representing the scan line portion of the object being imaged. The bits of data generated by the multi-segment linear photosensor assembly are commonly referred to as "picture elements" or "pixels." The term "pixel" is also sometimes used in the art to refer to individual photodetector elements or to the portions of an object that are imaged on these individual photodetector elements.
Sampling is a process by which the light emerging from an image being scanned is converted into an energy form, typically a voltage, as measured by a sensor, which is then digitized, read, and processed by a digital computer. Typically, the voltage or energy level of the measuring instrument will be substantially proportional to the light intensity emerging from an image.
Ideally, sensors placed over a completely dark image would therefore indicate an absence of energy, thereby ensuring that a subsequent light measurement would be highly accurate. In reality however, a phenomenon known as "dark current" exists. This "dark current" refers to a gradual accumulation of charge, or "sensor drift" by sensors positioned over a dark surface. When scanning real world surfaces, such dark current represents a source of error, or noise, to the overall scanning apparatus and tends to represent images as being lighter than they really are.
When sampling synchronously, sampling operations occur continuously. Accordingly, a disadvantage of the synchronous sampling approach is that many samples will be taken where no valuable image information is present, leading to a considerable waste of energy. In a battery powered hand-held device, such wasted energy may cause substantial inconvenience by causing the batteries to be rapidly drained.
The accuracy of a scanning operation demands accurate readings of light emission from a surface being scanned, as noted above. Accurate reconstruction of the image with digital data however, also requires that optical data acquired from the image be associated with positional data which accurately reflects the position at which such image data was collected. Various characteristics of optical sensors used in scanning operations may contribute to error in association of position data with image data.
Taking a sample of an image at a particular point may comprise turning on or pulsing a light source, such as a light emitting diode (LED), for a specific period of time and instructing appropriate signal processing equipment to convert a sensor value into digital data at the point on the image for which the sample is desired. A problem arises from the fact that a finite delay exists between issuing the command to turn on the light source and a point in time which represents the chronological center of the light pulse.
This delay causes the scanning system to process an image which is a finite distance away from the point at which sampling was sought and to incorrectly identify the location associated with the collected image data. There is a need in the art for a mechanism to correct any positional error due to the delay between the request and execution of an image sampling operation.
When the scanning instrument travels at a constant speed over the image being sampled, the sampling position error should at least be relatively constant among a number of measurements taken, making the overall sampling results susceptible to corrective action which is equally applicable to all measurements. A more difficult situation arises however, in the case where a scanning instrument travels at variable velocities throughout a scanning operation. The position error associated with each sample will be different and thus more difficult to correct.
Therefore, there is a need in the art for a sampling method which samples only where worthwhile information is located.
There is a further need in the art for a sampling method which does not waste energy by sampling where no valuable information is located.
There is a still further need in the art for a system and method for correcting the position error associated with sampling data arising from a delay between request for and execution of a sampling operation.